One of important functions of a welding robot is weaving welding. As is well known in the art, this weaving welding is a welding method of moving a welding torch along a weld line while swinging the same in a direction substantially perpendicular to the weld line.
As to workpieces (called "welded bodies" in this specification, and two welded bodies to be welded with each other are expressed by "first" and "second" respectively) serving as targets of welding, butt spaces between first and second welded bodies, groove widths and the like (these are generically named as "interspaces" between the welded bodies in this specification) are frequently nonuniform in the weld line direction because of dispersion of respective ones of cutting accuracy, bending accuracy and assembling accuracy caused by bend and distortion of materials and integrated errors thereof. Therefore, if weaving welding is performed while maintaining swing width of the welding torch, i.e., weaving width constant although the interspaces are ununiform as described above, excessive/insufficient welding is caused depending on portions and weld quality is extremely reduced.
In order to solve this problem, there have generally been proposed weld line profile control methods (for example, Japanese Patent Laid-Open Gazette No. 254979/1987 etc.) which can regularly perform good welding by automatically responding to changes of interspaces in weaving welding.
According to these proposed examples, welding voltages and welding currents are simultaneously measured while performing weaving welding, to detect weaving endpoints on the basis of changes of the welding voltages and the like. Assuming that an arc of a welding torch is controlled to maintain a constant welding voltage, for example, a current flowing in the welding torch during welding, i.e., a welding current, is reduced as the space between the leading edge portion of the welding torch and a groove face is widened, while being increased as the space therebetween is narrowed to the contrary. Therefore, a position where the current is abruptly changed is evaluated while continuously measuring the welding current, to judge the position as a weaving endpoint.
In the above proposed example, however, there is such a problem that it is impossible to satisfactorily perform welding (hereinafter referred to as "welding height constant control") in which a constant welding height is regularly ensured with respect to changes in groove width, as hereafter described. This is for the following reason:
As is generally known, the amount of melting of a welding wire per unit time is proportionate to the feed rate of the welding wire. Namely, if the feed rate of a welding wire is increased in the case of employing a power source of constant voltage characteristics such as normal consumable electrode type gas shield welding, the amount of melting of the welding wire per unit time is increased in order to maintain a constant arc voltage, and the welding current is increased as the result. If the feed rate of the welding wire is reduced to the contrary, the amount of melting of the welding wire per unit time is reduced in order to maintain a constant arc voltage, and the welding current is reduced as the result.
Therefore, two methods may be considered for welding height constant control. The first method is a method of changing the feed rate of the welding wire in response to the groove width while maintaining the travel speed of the torch constant. When welding height constant control is performed by this method, the welding current is significantly changed following changes of the feed rate of the welding wire, to easily cause a defect such as lack of joint penetration.
On the other hand, the second method is a method of changing the travel speed of the torch in response to the groove width while maintaining the feed rate of the welding wire constant (manufacturing an amount of melting per unit time constant). In order to carry out the second method, at least the position of a next weaving endpoint with respect to the current position of the welding torch must be already known, in relation to performance of speed control. In the above proposed example, however, the next weaving endpoint is detected while performing weaving operation, and hence it is impossible to perform welding height constant control by the second method.
Although it is not mentioned in the above description, weaving welding must be executed while maintaining a space (hereinafter referred to as "torch height") between the welding torch and the groove bottom surface constant in the aforementioned welding height constant control. Therefore, it is also necessary to control the torch height to be constant during weaving welding.